The present invention relates to a motor drive apparatus for driving motors such as ultrasonic motors.
Ultrasonic motors use ultrasonic vibrations as the driving force. In a progressive wave-type ultrasonic motor, a stator is comprised of an annular elastic body and a piezoelectric body which are attached to each other, and a rotor fixed to a drive shaft is pressured to contact the stator. The piezoelectric body is supplied with drive signals at a fixed frequency and in two phases, that is, in a sine wave and a cosine wave which are 90.degree. different in phase. The piezoelectric body produces mechanical vibrations in response to the two-phase drive signals to cause in the elastic body ultrasonic vibrations (progressive waves) which move in the annular direction along the elastic body. The progressive waves rotate the rotor pressure-contacted with the elastic body.
A drive apparatus which generates the drive signals is comprised of a microcomputer, an oscillator circuit, a switching control circuit, a drive signal generator circuit, a band pass filter and the like. In the drive apparatus, the drive signal generator circuit generates an alternating current (a.c.) voltage of a fixed oscillation frequency of the oscillator circuit. The drive apparatus applies the a.c. voltage to the piezoelectric body to drive the ultrasonic motor.
A rotation sensor is attached to the ultrasonic motor, and generates a rotation pulse signal every predetermined angular rotation of the motor. The microcomputer counts the rotation pulse signal to check whether a control object driven by the ultrasonic motor has reached to a predetermined position or whether the ultrasonic motor is operating normally.
The ultrasonic motor is used, for instance, in a tilting device and a telescopic device of a vehicle steering system. In this system, the d.c. voltage of a vehicle battery (about 12 V) is converted to the a.c. voltage (about 200 Vrms).
When an engine of the vehicle is driven by a starter motor during the operation of the ultrasonic motor, it may occur that the d.c. voltage of the battery cannot be boosted to the a.c. voltage required for the ultrasonic motor because the battery voltage tends to decrease during the engine starting (cranking) operation. Therefore, it is proposed to monitor the battery voltage by the microcomputer. The ultrasonic motor is stopped once when the battery voltage decreases below a predetermined level, and it is driven again after the battery voltage restores to the predetermined level.
For instance, when the battery voltage decreases below less than a predetermined level V1 (for instance, 9 V) at time t1 as shown by (A) in FIG. 9, the microcomputer detects the decrease in the battery voltage at time t2 after a detection delay time period T from time t1 as shown by (B) in FIG. 9. The voltage supply to the ultrasonic motor is stopped in response to the detection of voltage decrease as shown by (D) in FIG. 9. The ultrasonic motor is supplied with the voltage again at time t3, when the battery voltage restores or increases above a predetermined level V2 as shown by (A) in FIG. 9.
Further, the microcomputer calculates rotation speeds of the ultrasonic motor from the rotation pulse signal generated by the rotation sensor. The microcomputer determines that the ultrasonic motor is in the abnormal condition when the calculated rotation speed decreases below a predetermined level. The abnormal condition may be a motor lock or an unstable rotation, which occurs when a foreign matter is jammed in the motor. In this instance, the ultrasonic motor is not driven again even when the battery voltage restores the predetermined level.
When the battery voltage decreases as shown by (A) in FIG. 9, the rotation speed of the motor decreases. If the microcomputer detects the decrease in the voltage prior to the decrease in the rotation speed, the microcomputer fails to detect the abnormal condition as shown by (C) in FIG. 9. As a result, the ultrasonic motor is driven again when the battery voltage restores, even under the condition that the abnormal condition remains.
On the other hand, as shown in FIG. 10, when the microcomputer detects the abnormal condition, motor lock for instance, at time t2 before the detection of the decrease in the battery voltage at time t3 as shown in FIG. 10, the ultrasonic motor cannot be driven again even after the battery voltage restores the predetermined level V2 at time t4.
For the above reasons, it is likely to occur that the ultrasonic motor is disabled to drive the control object to predetermined positions when the starter motor is driven to start the engine during the operation of the ultrasonic motor.